In the past, it has been common to dispense liquid mercury into a lamp through an exhaust tubulation. Since this procedure has on occasion been considered an environmental hazard, as well as being wasteful, other techniques, involving the release of mercury from a solid after the lamp has been evacuated and sealed, have been employed.
These other techniques have involved the use of radio frequency (RF) induced currents in order to heat the mercury target. This has required the use of a metal antenna loop in order to intercept and convert the RF energy into an RF heating current.
In one such method the antenna took the form of a disintegration shield encircling the lamp oil. This shield contained an intermetallic Ti.sub.3 Hg alloy applied to one side of an oval-shaped ribbon loop made of a base metal such as nickel or stainless steel. The metal ribbon had a width of about 0.25 inches.
Another method of mercury dispensing employing the disintegration shield RF antenna principle was to position the mercury target across a gap in the ribbon shield. The mercury was contained in either a glass or metal capsule. In the case of the glass capsule a fine wire was either wrapped around the capsule or passed through it. The ends of the wire were then welded to each side of the shield gap to complete the loop current path. In the case of the metal capsule, the capsule itself is welded across the gap to complete the loop current path.
Previous dispensing techniques involving metal ribbon shields have relied on the heat generated by the RF current to raise the temperature of the metal loop or the wire or capsule across the shield gap to the level required for mercury release. The required temperature varied depending on the type of mercury target. The Ti.sub.3 Hg alloy releases mercury by thermal decomposition within a temperature range of 600.degree. C. to 1000.degree. C. The release time will be lower at the higher temperature. A release time of 25 seconds is achieved for a temperature of 900.degree. C. In the case of the glass capsule, the wire temperature required to crack the glass is about 1000.degree. C., and Hg release times are between 5 and 10 seconds. For the metal capsules, the mercury release is obtained when the vapor pressure within the capsule increases to the bursting point of the capsule design. This can vary considerably depending on the capsule material as well as the wall thickness. Release times of about 5 seconds have been reported using stainless steel capsule of 2-3 mil wall thickness.
All of these previous methods require the use of a closed loop metal antenna to convert the RF energy to RF heating current. This adds to the expense of the lamp and limits the minimum release time since a two-stage energy conversion process is required.